Gusseted bags have become very popular in the packaging industry. They are used extensively in the packaging of frozen foods as well as for coffee and cereal bags. Gusseted bags allow for expansion and can hold bulkier contents as compared to common pillow bags. However, this type of bag is notoriously difficult to seal. The difficulty lies in sealing across the multiple variations of layers that are unavoidable in creating a gusset.
The ends of a gusseted bag have several areas of folded material which forms the gussets. There may be areas with four layers of material on the outer edges, areas with two layers of material inboard thereof, and another area with four layers of material in the center of the gusset, resulting in a 4-2-4-2-4 layer end. While these folds are important to form the gussets, they create leak points at the transitions between the different areas of thickness. Bags with an integrated zippers create similar problems and often require a separate process to crush and seal the ends of the zipper.
By comparison, a pillow bag has relative uniformity with only two layers across a majority of the seal. In a pillow bag, a small four layer section resulting from the back fin seal is an insignificant percentage of the total cross seal. Pillow bags can be easily sealed using a variety of sealing technologies.
Heat is currently the preferred technology for gusseted cross seals. While creating a hermetic seal is unreliable with heat, the strength of the seal is good due to its wide width. Typical heat seal jaws produce cross seals that hold approximately 8″ Hg to 10″ Hg vacuum before leaking on a common coffee pouch.
Custom contour tapered seal anvils have been tried with heat, offering an improvement in seal quality. The idea of the contour is to accommodate the 4-2-4-2-4 layer variation of a gusseted bag by incorporating a corresponding height variation into the seal jaws. One company that specializes in this type of sealing tools reports sealing coffee bags with contoured seal anvil design produced seals holds an average of 16.1″ Hg before leaks occurred. One problem with the contoured/bowed anvil approach, however, is that the taper must be gradual and the location of the gussets and fins drifts from bag to bag in production. Further, there is occasionally a two layer section on the extreme outer edges of the seal that occurs if the gussets do not line up perfectly. Since the tools are contoured away from one another at this point, a leak in this location is almost certain.
The sealing of gusseted bags has been attempted with ultrasonics in the past without success. A hermetic seal can be created with ultrasonics using a traditional single seal bead, but it results in a fragile bond that is unable to withstand real-world abuse. When the seal experiences flexing or bulging from the internal product or a vacuum burst test, the transition point where four layers meets two layers becomes highly stressed and the seal quickly fails. Quite often the film bursts at this transition point, tearing through all layers instead of just creating a small leak path. This transition point failure has been an insurmountable hurdle for ultrasonic sealing of gussets. As a result, it is believed that there are currently no ultrasonic suppliers claiming to offer a solution for sealing gusseted bags.
It therefore desired to address these problems and provide an improved seal and method of sealing.